OSU researchers find cause of 2008 offshore earthquake swarms

NEWPORT, Ore. - A team of Oregon State University scientists has
solved the mystery behind an unusual swarm of earthquakes that
occurred off the Oregon coast in the spring of 2008 - a series of
faults in the Juan de Fuca plate that they didn't know existed.

The discovery of these faults about 140 miles off the central
Oregon coast, in association with the earthquake activity, suggests
that the tectonic plate off the Oregon shore is still actively
deforming, said Robert Dziak, an OSU marine geologist who works at
the university's Hatfield Marine Science Center in Newport.

"This pattern of earthquakes demonstrates that the Juan de Fuca
plate is continually moving and converging with North America at
the Cascadia Subduction Zone," Dziak said. "It isn't clear if the
swarms that occurred in 2008 represent normal stress release within
the plate, or if they are from deformation related to the Cascadia
Subduction Zone. We simply don't yet know."

Most of the earthquakes were of magnitude 3.0 to 4.0, the
scientists said, but there were a handful that exceeded magnitude
5.0. Few, if any, of these earthquakes would be felt on shore,
Dziak said, because they originate offshore within the deep ocean
floor. The Cascadia Subduction Zone is of particular interest
because the region has experienced several enormous earthquakes
over the past 10,000 years - the last of which occurred about 300
years ago.

The intense earthquake swarm last year began on March 30 about
140 miles southwest of Newport and was one of the more unusual
events detected by Dziak and his colleagues in 17 years of
monitoring using sensitive undersea hydrophones. The swarm was
considered unusual because it began inside the Juan de Fuca plate
and not along the boundary between the Juan de Fuca and Pacific
plates, where most earthquake activity takes place.

Then after 10 days, the swarm stopped - but not for long. Three
distinct clusters of quakes soon followed, beginning with a series
of small tremors along the Blanco Transform Fault - the boundary
between the two plates - and concluding with a frenzy of seismic
activity along the Gorda Ridge, which produced more than 1,000
earthquakes in just five days. This swarm was of special interest
to scientists, not just because of the sheer volume of quakes, but
because of its proximity to an eruption on the seafloor discovered
in 1996.

Dziak said the two-month swarm represented a plate motion event,
beginning within the Juan de Fuca plate, then moving east and
south, and finally culminating in seafloor spreading activity that
likely produced magma intruding beneath the seafloor.

"We were able to monitor the spatial progression of the swarm
within the plate and along its boundaries," Dziak said, "but we
don't yet completely understand how they are related and what
triggers the sequence. But it is interesting that the stress
release within the plate could trigger swarms of earthquakes on the
plate boundaries."

During the two-month spree last spring, the OSU scientists
recorded more than 1,600 earthquakes using an array of hydrophones
called the Sound Surveillance System (SOSUS), made available by the
U.S. Navy. This SOSUS network originally was used during the
decades of the Cold War to monitor submarine activity in the
northern Pacific Ocean. As the Cold War ebbed, these and other
unique military assets were offered to civilian researchers
performing environmental studies, Dziak said.

When the researchers first detected the swarm, they mobilized
the OSU research vessel Wecoma on a trip led by Ron Greene to take
water samples in the earthquake zone and look at the chemical
signature of the water for signs of volcanic activity. In
September, Hatfield researcher Susan Merle returned aboard the R/V
Melville and performed a multi-beam sonar survey to produce new
maps of the seafloor and it was during this cruise that the new
fault system was discovered.

"From aboard the ship, we discovered one area where there was a
20-meter displacement of the seafloor and deformed sediments, which
is a direct indication of faulting," said Merle, a senior faculty
research assistant at OSU. Merle, Greene and Dziak are all
affiliated with the Cooperative Institute for Marine Resources
Studies (CIMRS), a joint OSU/NOAA venture.

An additional high-resolution seafloor survey of the eastern
Blanco Transform Fault was performed last summer by the NOAA ship
Okeanos Explorer.

This isn't the first time the researchers have recorded
earthquake swarms off the Oregon coast. There have been a total of
eight swarms over the past dozen years, Dziak said, the first seven
of which likely were the result of volcanic activity on the Juan de
Fuca and Gorda ridges. The 2008 swarm originated within the plate,
where the newly discovered faults lie and affected a large area of
the plate and its boundaries.

"The discovery sheds some new light on the structure and seismic
processes of the region," Dziak said, "and suggests that
deformation within the plate and earthquakes along its boundaries
may be more interrelated than we though. It also underscores the
importance of having ships available to go to normally inaccessible
areas of the deep-ocean for research that addresses societal
concerns."

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